1. Field of the Invention
This invention relates to concentrator solar cell modules and fabrication techniques, and more particularly to panels that use a large number of small size solar cells and light concentrating optics.
2. Description of the Related Art
Prior concentrator solar cell modules have generally employed a matrix of relatively large area solar cells, typically at least 12.5 mm per side for a square cell. The cells are individually mounted by hand onto a support panel, with output electrical lines also hand wired. A solar cell assembly of this type is described in Richards, "Sandia's Baseline 3 Photovoltaic Concentrator Module", Proceedings of the 20th IEEE Photovoltaic Specialists Conference., Sep. 25-30, 1988, pages 1318-1323. The need for hand assembly adds significantly to the cost of manufacture. In addition, during cell manufacturing a single point defect can spoil an entire cell. Furthermore, the manufacturing yield for the cells is limited because of their relatively large size.
Solar concentrator lenses that can be several hundred times the cell areas are mounted above the cells to concentrate received solar radiation onto their respective cells below. Since higher degrees of concentration yield higher conversion efficiencies, it is desirable that the concentractor lenses be kept relatively large in size. However, this also increases their focal lengths, requiring a depth on the order of a foot or more for the overall assembly. The large cell and lens areas result in panels that are relatively large and heavy; this adds to the difficulty of securely mounting them and especially of guiding their movements in a solar tracking mode.
A photovoltaic assembly with smaller solar cells, on the order of 3 mm per side, has been introduced by WattSun Corporation of Albuquerque, N.M. In this system the solar cells are hand bonded directly to a common copper substrate in a parallel circuit configuration. The concentration ratio is limited to about 30:1, rather than the desired range of several hundred to one, because the assembly methods employed do not allow for the high quality alignment of the solar cells. In addition, the WattSun approach does not permit the use of secondary focusing optics, which are necessary in systems with a light concentration over 100:1. The system is based upon silicon rather than high efficiency GaAs cells, since it is not economically compatible with GaAs. Thus, while the WattSun approach uses a larger number of smaller cells in place of the prior smaller number of large cells to obtain advantages in overall panel size and cell yield, its conversion efficiency is limited and it still employs costly manual fabrication techniques. Furthermore, the cells can only be connected in parallel to accumulate currents; the system is not compatible with a series connection for accumulating cell voltages, a configuration that is advantageous for the operation of the system.